Vol. 631
In section 5. Galactic structure, stellar clusters and populations

The Milky Way has no in-situ halo other than the heated thick disc. Composition of the stellar halo and age-dating the last significant merger with Gaia DR2 and APOGEE

Gaia DR2 data have revealed a wealth of accreted halo stars within a few kiloparsecs of the Sun. These stars have been postulated to originate in a single progenitor galaxy that merged with our Galaxy, and they are now a significant component of our halo. In this paper, the authors carefully study this population by combining astrometric data from Gaia DR2 with elemental abundances from APOGEE DR14 to characterize the kinematics and chemistry of stars in the Galactic halo down to [Fe/H] >~ -2. They find that the accreted stellar population dominates the halo, representing about 60% of all stars at [Fe/H] < -1; the remaining 40% are made up of thick disk stars. They also find a stellar kinematic imprint left by this accretion event, which heated the old Galactic disk. They are able to use this to age-date it to between 9 Gyr and 11 Gyr ago. This heating process resulted in an important fraction of stars with abundances typical of the thick disk that are scattered into the Galactic halo and that dominate the stellar halo of the inner Galaxy. The new picture emerging from this study is one where the standard non-rotating in situ halo population, the collapsed halo, seems to be more elusive than ever.